It is known that the internal resistance of a fuel cell influences humidity coefficient of an electrolytic film in the fuel cell, and when a small amount of the moisture is present in the fuel cell and the electrolytic film is relatively dry, the internal resistance increases and the output voltage of the fuel cell decreases. On the other hand, when an excessive amount of the moisture is present in the fuel cell, the electrode of the fuel cell is covered with the moisture, and in consequence, diffusion of oxygen and hydrogen as reactants is inhibited, and thus the output voltage decreases.
To operate the fuel cell with a high efficiency, the amount of the moisture in the fuel cell needs to be controlled in an optimum manner. There is a correlation between the amount of the moisture in the fuel cell and the impedance of the fuel cell. At present, the impedance of the fuel cell is measured by an AC impedance method, whereby the moisture status in the fuel cell is indirectly grasped.
For example, in Patent Document 1 described hereinafter, a method is disclosed in which a sinusoidal signal (impedance measuring signal) having an arbitrary frequency is applied to (superimposed on) an output signal of the fuel cell to measure the impedance in this case, whereby the amount of the moisture in the fuel cell is grasped in real time.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-86220